Preparation and crystallization of hollow α-Fe2O3 microspheres following the gas-bubble template method

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Valladares, LDLS 
León Félix, L 
Espinoza Suarez, SM 
Bustamante Dominguez, AG 
Mitrelias, T 

In this work we report the formation of hollow α-Fe2O3 (hematite) microspheres by the gas-bubble template method. This technique is simple and it does not require hard templates, surfactants, special conditions of atmosphere or complex steps. After reacting Fe(NO3)3.9H2O and citric acid in water by sol-gel, the precursor was annealed in air at different temperatures between 180 and 600 ºC. Annealing at 550 and 600 ºC generates bubbles on the melt which crystallize and oxidizes to form hematite hollow spheres after condensation. The morphology and crystal evolution are studied by means of X-ray diffraction and scanning electron microscopy. We found that after annealing at 250-400 ºC, the sample consist of a mixture of magnetite, maghemite and hematite. Single hematite phase in the form of hollow microspheres is obtained after annealing at 500 and 600 ºC. The crystallization and crystal size of the hematite shells increase with annealing temperature. A possible mechanism for hollow sphere formation is presented.

Nanostructures, Oxides, Crystal growth, X-ray diffraction, Magnetic materials
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Materials Chemistry and Physics
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Elsevier BV
Engineering and Physical Sciences Research Council (EP/J003638/1)
This work was supported by the Engineering and Physical Science Research Council (EPSRC No. EP/J003638/1). The work in Peru has been supported by CONCYTEC. The work in Brazil was supported by CNPq (307552/2012-8), CAPES (PNPD- 230.007518/2011-11) and FACEPE (APQ-0589-1.05/08).